Method of providing conductive tracks on a printed circurt and apparatus for use in carrying out the method

ABSTRACT

This invention is a method of providing conductive tracks on a printed circuit including coating a substrate carrying printed tracks with an electro-plating solution with a tool which provides a first electrode of an electro-plating circuit and a second electrode provided by the tracks which are to be electroplated, and a tool suitable for use in electro-plating electrically conductive regions of a substrate, the tool including an absorptive member in which plating solution can be carried; a first electrode of an electro-plating circuit adapted to make electrical contact with plating solution carried by the absorptive member; and at least one tool second electrode electrically insulated from the first electrode and spaced from the absorptive member, the tool second electrode being so positioned that as the absorptive member is wiped across a surface of a substrate, the second electrode contactor can be wiped across the surface of the substrate to contact electrically conductive regions of the substrate to form a second electrode of the electro-plating circuit therewith.

RELATED APPLICATION

[0001] This is a continuation of International Application No.PCT/GB99/00890, with an international filing date of Apr. 6, 1999, whichis based on British Patent Application No. 9807280.4, filed Apr. 6,1998.

FIELD OF THE INVENTION

[0002] This invention is concerned with a method of providing conductivetracks on a printed circuit and an apparatus suitable for use incarrying out the method.

BACKGROUND

[0003] A well known and convenient way of producing printed circuitsincludes the printing of electrically conductive tracks on a substrate,for example, using screen printing techniques. The tracks are printedusing an electrically conductive ink which typically comprises apolymeric material having electrically conductive particles, forexample, copper, silver or other suitable metal, dispersed in thepolymeric composition. The polymeric materials are typically cured to asolid condition by subjecting them to radiation, for example, infra-redradiation or ultra-violet light.

[0004] Although the conductive inks which are used have a sufficientelectrical conductivity for use in certain circumstances, the electricalconductivity has in no case been as great as the electrical conductivityof copper or other highly conductive metals. Even the conductive inkswith the best performance have electrical conductivities which in mostcases are only one tenth of the electrical conductivity of copper. Ithas been proposed to enhance the conductivity of the conductive tracksas continuous printed circuits by electro-plating the tracks with asuitable layer of metal, for example, copper but, in order to carry outelectro-plating, it is necessary to have a continuous electricalcircuit: that is not conveniently possible where the printed conductivetracks of a printed circuit are discrete and are not connected with oneanother. Furthermore, electro-plating has required the immersion of thesubstrate carrying the printed conductive tracks in a bath of suitableelectro-plating solution: that technique restricts the substrates whichcan be subjected to such an immersion electro-plating technique—forexample—paper based substrates are generally unsuitable as they willtend to be attacked and softened by the electro-plating solution. It hasalso been proposed to provide a more conductive coating on conductivetracks of printed inks by electroless plating. Electroless platinginvolves the use of plating solutions which are less stable than thosecommonly used in electro-plating and the process is less readilycontrolled. Furthermore, electroless plating still requires thesubstrate to be immersed in the plating solution with the consequentpossibilities of attack of the substrate as well as being restricted inthe thickness of deposit.

[0005] Thus, it would be advantageous to provide an improved method ofproviding conductive tracks on a printed circuit by electro-plating. Itwould be further advantageous to provide an improved printed circuit.

SUMMARY OF THE INVENTION

[0006] In one aspect the invention relates to a method of providingconductive tracks on a printed circuit including coating a substratecarrying printed tracks with an electro-plating solution with a toolwhich provides a first electrode of an electro-plating circuit and asecond electrode provided by the tracks which are to be electroplated.

[0007] In another aspect the invention relates to a tool suitable foruse in electro-plating electrically conductive regions of a substrate,the tool including an absorptive member in which plating solution can becarried, a first electrode of an electro-plating circuit adapted to makeelectrical contact with plating solution carried by the absorptivemember, and at least one tool second electrode electrically insulatedfrom the first electrode and spaced from the absorptive member, the toolsecond electrode being so positioned that as the absorptive member iswiped across a surface of a substrate, the second electrode contactorcan be wiped across the surface of the substrate to contact electricallyconductive regions of the substrate to form a second electrode of theelectro-plating circuit therewith.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In the accompanying drawings:

[0009]FIG. 1 is a diagrammatic plan view of a printed circuit havingdiscrete conductive tracks showing a tool in accordance with theinvention; and

[0010]FIG. 2 is a diagrammatic side view of a tool suitable for use incarrying out a method in accordance with the invention and itselfillustrating the invention in one aspect.

DETAILED DESCRIPTION OF THE INVENTION

[0011] In one aspect the invention provides a method of providingconductive tracks on a printed circuit by electro-plating conductivetracks which have been produced by printing them onto a substrate,comprising coating a substrate carrying the printed tracks with anelectro-plating solution using a tool which provides a first electrodeof an electro-plating circuit and with a second electrode provided bythe tracks which are to be electroplated.

[0012] In carrying out a method in accordance with the invention the inkforming the conductive tracks preferably comprises a cured polymercomposition loaded with electrically conductive particles.

[0013] Conveniently in carrying out a method in accordance with theinvention, a first pole of an electro-plating circuit is connected tothe first electrode and a second, opposite pole of the plating circuitis connected to the second electrode.

[0014] Preferably in carrying out a method in accordance with theinvention use is made of apparatus including a tool which comprises anabsorptive member in which the plating solution can be carried, in whichthe first electrode is in electrical connection with plating solutioncarried by the absorptive member, and in which the coating of platingsolution is applied to the substrate by wiping the absorptive memberover the substrate. Conveniently the tool also comprises a secondelectrode, electrically insulated from the first electrode and spacedfrom the absorptive member, in which the second electrode is adapted tobe wiped across the surface of the substrate as the absorptive member iswiped across the surface of the substrate.

[0015] In another method in accordance with the invention the conductivetracks may be treated with a scanning electron beam to ionize the tracksand create an opposite polarity to the polarity of the first electrode.

[0016] Conveniently in carrying out a method in accordance with theinvention the plating solution comprises copper sulphate; however, anysuitable electro-plating solution which can be carried by the absorptivemember may be used. In carrying out a method in accordance with theinvention the conductive tracks are conveniently coated with platingsolution sufficient to deposit a layer of copper on the tracks which isof a desired thickness, typical about 2 microns in thickness.

[0017] In another aspect the invention may be considered to provide atool suitable for use in electro-plating electrically conductive regionsof a substrate comprising an absorptive member in which the platingsolution can be carried, a first electrode adapted to make electricalcontact with a plating solution carried by the absorptive member, and atleast one second electrode electrically insulated from the firstelectrode and spaced from the absorptive member, the second electrodebeing so positioned that as the absorptive member is wiped across asurface of a substrate the second electrode can be wiped across the saidsurface of the substrate.

[0018] In carrying a method in accordance with the invention, use ispreferably made of a tool in accordance with the invention.

[0019] The absorptive member of a tool in accordance with the inventionmay be provided by any suitable means within which the plating solutionmay be absorbed—for example the absorptive member may comprise a brushor a flexible foam material having interconnected pores.

[0020] Suitably a tool in accordance with the invention comprises meansto feed a supply of plating solution to the absorptive member.

[0021] A preferred tool in accordance with the invention comprises twoelectrically connected second electrodes mounted such that as theabsorptive member is wiped across the surface of a substrate one of thesecond electrodes leads the absorptive member and the other secondelectrode trails the absorptive member. Conveniently, the or each secondelectrode is provided by a flexed electrically conductive blade, forexample a flexible metal blade, e.g. of copper.

[0022] In another aspect the invention may be considered to provide aprinted circuit comprising a plurality of discrete conductive tracks,each track comprising a layer of a cured electrically conductive ink onan electrically insulating substrate and a layer of conductive metaldeposited on the cured ink by electro-plating.

[0023] In carrying a method in accordance with the invention it is notnecessary to immerse the substrate carrying the conductive tracks in aplating bath. The quantity of plating solution which comes into contactwith the substrate is very small and the method can in effect beregarded as a substantially dry electro-plating method: consequently, itis possible to electro-plate tracks on substrates which cannot beelectro-plated by an immersion system. Furthermore, in carrying out thepreferred method in accordance with the invention, it is not necessarythat the conductive tracks be continuous, the use of a tool inaccordance with the invention or the scanning electron beam techniqueenables discreet electrically conductive tracks to be electro-plated. Amethod in accordance with the invention can be controlled to provide anaccurate plating thickness: the thickness of a layer applied byelectro-plating is a function of current and time for which theelectro-plating solution is in contact with the region to be plated. Itis therefore possible to electro-plate regions of a printed circuit toprovide electro-plated regions of different thickness thereby permittingadjustment of the resistance of regions of the printed circuit toprovide resistors of the circuit. Such a system is particularlyconveniently carried out using the scanning electron beam which can betargeted accurately on to a specific conductive track and moved rapidlyto achieve the desired conductivity.

[0024] Although in carrying out a method in accordance with theinvention the tool by which the plating solution is applied may be handheld, it is preferable to mount the tool in a suitable machine which maywipe the tool across the surface of the substrate carrying theconductive tracks.

[0025] A particularly preferred embodiment of aspects of the inventionis shown in FIG. 1. A printed circuit 10 comprises a substrate 12 onwhich are printed a plurality of conductive tracks 14. The tracks 14 areprinted onto the surface of the substrate 12 using a screen printingtechnique, the screen printing ink being provided by a polymericcomposition loaded with electrically conductive silver particles andcured by exposure to ultra-violet light to provide a pattern of discreteconductive tracks. The tracks as initially printed using the UV curableink are of relatively low conductivity.

[0026] In carrying out the illustrative method, of enhancing theconductivity of conductive tracks which have been produced by printingthem onto the substrate 12, the substrate carrying the printed tracks 14is coated with an electro-plating solution using a tool 16 which isindicated in chain dot line on FIG. 1 and which is showndiagrammatically from the end in FIG. 2, with part broken away.

[0027] The tool 16 comprises an absorptive member 18 provided by a bodyof flexible foam material having interconnecting pores. The tool 16further comprises a first electrode (not shown) which projects into theabsorptive member 18. The tool further comprises a pair of secondelectrodes 20 mounted at either side of the absorptive member 18,closely adjacent to a tip portion 22 of the absorptive member 18 butspaced from the tip portion 22 and electrically insulated from the firstelectrode and from the absorptive member.

[0028] Each of the second electrodes is provided by a flexibleelectrically conductive blade member which is made of any suitable metalmaterial for example a copper alloy.

[0029] In carrying out the illustrative method which can be regarded asa specialized and innovative application of the known brush platingmethod, the absorptive member 18 is impregnated with an electro-platingsolution so that the first electrode makes electrical contact with thesolution. The tool 16 is then moved into contact with the surface of thesubstrate 12, with the tip region 22 of the absorptive member 18pressing lightly on the surface of the substrate 12 and with the secondelectrodes likewise lightly contacting the surface of the substrate 12.

[0030] After the tool 16 has been brought into contact with the surfaceof the substrate 12 it is wiped along the substrate 12 in the directionindicated by the arrow A on FIGS. 1 and 2. As can be seen viewing FIG. 1the tool 16 extends completely across the substrate 16. Electro-platingsolution is supplied to the absorptive member 18 in sufficient quantitythat, as the tool 16 is wiped across the surface of the substrate acoating of electro-plating solution is wiped by the tool 16 across thesurface of the substrate. The first electrode is connected to a positivepole of an electro-plating circuit to provide the anode, whilst both ofthe second electrodes 20 are connected to the negative pole of theelectro-plating-plating circuit to provide the cathode. Thus, as thetool 16 is wiped across the surface of the substrate, the conductivetracks 14 are electro-plated. The electrical current supplied by theelectro-plating circuit, in conjunction with the speed of travel of thetool across the surface of the substrate provides a control of theamount of electro-plating metal which is deposited on the conductivetracks and is suitably controlled to provide a layer of metalelectro-plated on to the conductive tracks, which is of a desiredthickness, normally between about 10 and 15 microns in thickness. Anyappropriate electro-plating may be used but a common copper sulphateelectro-plating solution may be appropriate.

[0031] When the surface of the substrate 12 has been treated by the tool16, any excess electro-plating solution may be rinsed from the surfaceof the substrate 12, if necessary.

[0032] As the lines printed on to the substrate 12 by the curable inkmay be relatively fragile the pressure exerted by the tools 16 on thesubstrate should be very light, just sufficient to apply the necessaryelectro-plating solution and to make electrical contact between thesecond electrodes 20 and the conductive tracks 14.

[0033] The illustrative method provides a readily controlled method ofelectro-plating discrete conductive tracks carried on the surface of aprinted circuit substrate. As the substrate is not immersed in anelectro-plating bath, and the method is a substantially “dry” method ofelectro-plating, with only small amounts of electro-plating solutioncoming into contact with the substrate, it is possible to electro-platesubstrates which would be adversely affected by immersion in anelectro-plating bath. Furthermore, the method allows electro-plating ofdiscrete conductive tracks which has not hitherto been possible in anyconvenient manner.

What is claimed is:
 1. A method of providing conductive tracks on aprinted circuit comprising coating a substrate carrying printed trackswith an electro-plating solution with a tool which provides a firstelectrode of an electro-plating circuit and a second electrode providedby the tracks which are to be electroplated.
 2. A method according toclaim 1 in which ink forming the conductive tracks comprises a curedpolymer composition loaded with electrically conductive particles.
 3. Amethod according to claim 1 wherein a first pole of the electro-platingcircuit is connected to the first electrode and a second, opposite, poleof the plating circuit is connected to the second electrode.
 4. A methodaccording to claim 1 in which the tool comprises an absorptive member inwhich the plating solution is carried, in which the first electrode isin electrical connection with the plating solution carried by theabsorptive member, and in which the coating of plating solution isapplied to the substrate by wiping the absorptive member over thesubstrate.
 5. A method according to claim 4 in which the tool alsocomprises the second electrode, electrically insulated from the firstelectrode and spaced from the absorptive member, in which the toolsecond electrode is adapted to be wiped across the surface of thesubstrate as the absorptive member is wiped across the surface of thesubstrate.
 6. A method according to claim 1 wherein the conductivetracks are treated with a scanning electron beam to ionize the tracksand create an opposite polarity to the polarity of the first electrode.7. A method according to claim 1 wherein the plating solution comprisescopper sulphate.
 8. A method according to claim 7 wherein the conductivetracks are coated with plating solution to deposit a layer of copper onthe tracks which is about 20 microns thick.
 9. A tool suitable for usein electro-plating electrically conductive regions of a substrate, saidtool comprising: an absorptive member in which plating solution can becarried; a first electrode of an electro-plating circuit adapted to makeelectrical contact with plating solution carried by the absorptivemember; and at least one tool second electrode electrically insulatedfrom the first electrode and spaced from the absorptive member, the toolsecond electrode being so positioned that as the absorptive member iswiped across a surface of a substrate, the second electrode contactorcan be wiped across the said surface of the substrate to contactelectrically conductive regions of the substrate to form a secondelectrode of the electro-plating circuit therewith.
 10. A tool accordingto claim 9 wherein the absorptive member is a brush.
 11. A toolaccording to claim 9 wherein the absorptive member comprises a flexiblefoam material having interconnecting pores.
 12. A tool according toclaim 10 further comprising means to feed a supply of plating solutionto the absorptive member.
 13. A tool according to claim 10 comprisingtwo electrically connected tool second electrodes mounted such that asthe absorptive member is wiped across the surface of a substrate one ofthe tool second electrodes leads the absorptive member and the othertool second electrode, trails the absorptive member.
 14. A toolaccording to claim 10 wherein the or each tool second electrode isprovided by a flexible electrically conductive blade.
 15. A printedcircuit comprising a plurality of discrete conductive tracks, each trackcomprising a layer of a cured electrically conductive ink on anelectrically insulating substrate and a layer of conductive metaldeposited on the cured ink by electro-plating.
 16. A method according toclaim 2 wherein a first pole of the electro-plating circuit is connectedto the first electrode and a second, opposite, pole of the platingcircuit is connected to the second electrode.
 17. A method according toclaim 2 wherein the conductive tracks are treated with a scanningelectron beam to ionize the tracks and create an opposite polarity tothe polarity of the first electrode.
 18. A tool according to claim 10further comprising means to feed a supply of plating solution to theabsorptive member.
 19. A tool according to claim 11 further comprisingmeans to feed a supply of plating solution to the absorptive member.